PARIS, March 13 (Reuters) - The OECD said on Thursday it had postponed all activities related to Russia's accession to the organisation following a request from its 34 members.

The move by the organisation, which includes economic heavyweights such as the United States, Germany, France, Japan and Mexico, is the latest effort to apply pressure on Moscow ahead of a referendum on Sunday in which Ukraine's southern region of Crimea could vote to secede and join Russia.

The Paris-based Organisation for Economic Cooperation and Development did not give further details, but according to its website the grouping opened discussions with Russia in 2007 for membership of the organisation.

The OECD originally turned down a request in 1996 from Russia to become a member under President Boris Yeltsin.

The accession procedure is complex and can be long, as it involves a series of examinations to assess a country's ability to meet OECD standards in a wide range of policy areas.

During membership discussions, countries must demonstrate or bring their policies up to OECD standards in a range of areas such as tax, the environment and statistics.

Gaining access to the organisation is considered recognition that a country's economy and development level have reached an important mark.

The OECD also said its members had agreed to strengthen cooperation with Ukraine to help it address the "public policy challenges it faces".

Germany's Angela Merkel warned Moscow on Thursday that it risked "massive" political and economic damage if it refused to change course on Ukraine, saying Western leaders were united in their readiness to impose sanctions on Russia if necessary. (Reporting by Leigh Thomas; Writing by John Irish; Editing by James Regan and Andrew Callus)